The invention relates to an operating method for a cryo-compressed tank, in which may be stored the cryogenic hydrogen for supplying a consumer, in particular an internal combustion engine and/or a fuel cell of a motor vehicle under supercritical pressure at 13 bar or more. In this case, in order to compensate for the loss of pressure resulting from the removal of hydrogen from the cryo-compressed tank, the removed hydrogen that has been heated in a heat exchanger is conveyed to a heat exchanger, provided in the cryo-compressed tank, by way of a tank pressure regulating valve and a branch line, which branches off of a supply line leading to the consumer, and, after flowing through the heat exchanger, is introduced into the supply line downstream of the branching off of the branch line. With respect to the known prior art, reference is made especially to the German patent applications 10 2007 011 530.1 (having U.S. counterpart application Ser. No. 12/546,998, filed Aug. 25, 2009) and 10 2007 011 742.8, which were not published earlier. In addition, reference is also made to the U.S. Pat. No. 6,708,502 B1 and to the technology applied in the “hydrogen 7” vehicle of the assignee of the present patent application.
The known prior art in the so-called “hydrogen 7” vehicle, which is equipped with a so-called cryo tank for storing cryogenic hydrogen (for supplying the vehicle drive unit, configured as an internal combustion engine), is a “sub-critical” storage vessel (as the cryo tank), which consists of a metal inner tank, a metal outer tank, and a vacuum super insulation, which is sandwiched between the two tanks and which serves to reduce the introduction of heat into the inner tank. The typical operating pressure of this storage vessel ranges from 1 bar absolute to 10 bar absolute, and the operating temperatures in the so-called “standard cryogenic operation” range from 20 K (Kelvin) to approximately 30 K. That is, the cryogenic hydrogen, contained in the storage vessel and/or in the inner tank of the same, exhibits these physical values, which lie in the so-called sub-critical range in the pressure-density diagram of the hydrogen.
The so-called cryo-compressed storage system represents an additional known prior art, for which reason reference is made to the aforementioned U.S. Pat. No. 6,708,502 B1, which describes different types of insulated compressed storage systems for cryogenic storage mediums with inner and outer diffusion barriers, which envelop an inner tank made of carbon fiber reinforced plastic. According to this prior art, the described so-called cryo-compressed storage system can be filled with warm pressurized gas at 350 bar and at low storage capacity or as an alternative with liquid hydrogen at low pressure of approximately 1 bar (absolute) at higher storage capacity.
The two German patent applications, which were not published earlier and are described above, disclose that in a so-called cryo-compressed tank, which can be designed like the cryo-compressed storage system described in the previous paragraph, supercooled cryogenic hydrogen is stored under supercritical pressure, that is, at least at 13 bar absolute, or at a significantly higher pressure, that is, at 150 bar or more, and/or during filling of the tank, such hydrogen can be filled into a suitable cryo-compressed tank under supercritical pressure. With respect to the significant advantages of this technology, in particular at relatively high pressure values, reference is made to the statements made in the cited documents that have not been published earlier.
It may be desirable to be able to remove the hydrogen for supplying the consumer at a certain minimum pressure level. In the aforementioned “hydrogen 7” vehicle, this minimum pressure level in the tank is set by way of a targeted introduction of heat into the cryo tank. This heat input is achieved in that after a sub quantity of the hydrogen, removed from the tank, has been heated in an external heat exchanger, it is conveyed as the heat carrying medium through an (internal) heat exchanger, provided in the cryo tank. At the same time heat is dissipated to the hydrogen stored in the cryo tank. After flowing through this internal heat exchanger, the heat carrying hydrogen is returned again to the supply line leading to the consumer. Since the maximum pressure, which may prevail in this known cryo tank, is relatively low, only a continuously adjusted subquantity of the removed hydrogen, which was heated in an external heat exchanger, may be fed as the heat carrying medium to the (internal) heat exchanger, provided in the cryo tank. Thus, for an adjusted throttling and/or metering of the quantity of heat carrying medium, there is a tank pressure regulating valve, which is actuated as a function of the internal pressure of the cryo tank. However, a suitable continuous actuation of this regulating valve is relatively expensive.
For a cryo-compressed tank, in which the cryogenic hydrogen can be stored at absolute pressure values in a magnitude of 13 bar or more—that is, up to, for example, 350 bar, as mentioned in the above described U.S. Pat. No. 6,708,502 B1, the object is to provide an operating method that is simpler than that described in said prior art.
The solution to this problem is characterized in that over a period of time that significantly exceeds the cycle times of a conventional frequency valve, either the removed amount of hydrogen, which has been heated, is guided without limitation into the heat exchanger, provided in the cryo-compressed tank, the tank pressure regulating valve being completely open, or no return of the heated hydrogen into the heat exchanger, provided in the cryo-compressed tank, occurs at all; and that downstream of the branching off of the branch line, the supply line has a pressure regulating unit, which ensures that irrespective of the changes in the pressure in the supply line caused upstream of this pressure regulating unit by switching the tank pressure regulating valve, a sufficient and continuous supply of hydrogen to the consumer at the pressure required is guaranteed.
Aware of the fact that a cryo-compressed tank, which allows internal pressure values in a magnitude of 150 bar and more (for example, the cited 350 bar), permits far greater fluctuations in the internal pressure of the tank than, for example, a cryo tank of the above described “hydrogen 7” vehicle, the invention proposes, instead of the known expensive control element of the tank internal pressure for the tank pressure regulating valve, provided to this end, a simple so-called “black and white operating mode” with long cycle times, to the effect that this tank pressure regulating valve, which continues to be actuated as a function of the internal pressure in the cryo-compressed tank, is alternatingly either totally closed or totally open over a relatively long period of time, which significantly exceeds the typical cycle times of a conventional frequency valve. Therefore, the range of the tank pressure threshold values of a control element, which actuates the tank pressure regulating valve, that is, opens and/or closes said valve, must be set correspondingly wide. It has been recognized that the pressure fluctuations, which are automatically larger and/or relatively large with such a control strategy and which occur in a supply line conveying the hydrogen taken from the tank, can be tolerated, if downstream of the branching off of the branch line from the supply line, this supply line has a pressure regulating unit, as stated above.
In particular, it has been recognized that if the cryo-compressed tank is filled just partially to a significant extent at a so-called partial filling degree, for example, is filled up to half or filled less than 30%, then the tank pressure regulating valve can remain continuously open, especially if chronologically thereafter the pressure value in the cryo-compressed tank rises. This pressure value exceeds an upper threshold value, cited in the previous paragraph, for the actuation of the tank pressure regulating valve, in more concise terms, for closing this valve. It was recognized that beyond a certain system-specific partial filling degree despite the continuously open tank pressure regulating valve and, thus, noteworthy introduction of heat by way of the internal heat exchanger into the tank itself, even an additional, not completely avoidable heat input by way of the insulation of the tank can not produce a critical pressure increase in the cryo-compressed tank that could exceed a design specific limit value for the internal pressure of the tank. Then a large amount of heat is introduced by way of the continuously open tank pressure regulating valve into the cryo-compressed tank such that a prolonged supply of hydrogen to the consumer at the required higher pressure is possible than if on reaching this system specific partial filling degree the tank pressure regulating valve does not remain continuously open, that is, not for a prolonged period of time. The latter and/or the correspondingly proposed control strategy that serves to actuate the tank pressure regulating valve is referred to below as a reserve control and is explained below with reference to the accompanying FIG. 2.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.